Blocking oscillator



Feb. 19, 1952 E. c. DlLL 2,586,310

BLOCKING OSCILLATOR Filed Feb. 28, 1948 WITNESSES: INVENTOR 55a 77% Eve/eff CDi/l.

' BY w. 4 ATTORN Patented Feb. 19 1952 BLOCKING OSCILLATOR Everett C. Dill, Arlington, S. Dak., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application February 28, 1948, Serial No. 11,919

3 Claims.

My invention relates to harmonic generators employing electron tubes, and inv particular relates to a set of circuit connections to enable electron tubes of the so-called blocking-oscil lator type to generate periodic voltage pulses of extremely short duration.

For a number of different purposes now familiar in the field of high frequency electrical techniques, it is desirable to be able to generate voltage pulses, which may recur at intervals in the order of one-millionth of a second, and in which the voltage pulses themselves have an extremely short duration.

One example of such a purpose is in a device referred to by the name of a harmonic generator," in which short pulses are produced at fixed intervals separated from each other by a substantially longer intervening period of zero signal. In a system of this type, the practice has been to employ what is known as a single swing blocking oscillator to generate the voltage pulses. If the voltage pulses occur at fixed intervals which we can denote as ,f, the harmonics available from such voltage pulses will be of the series (2f+3f+4f+5f+ +nf), where n is a whole integer. on the slope of the leading edge of the pulse; hence the faster the voltage pulse increases, the higher will be whole integer denoted as n. It is obvious that if a pulse of extremely short duration is produced, it will of necessity have a leading edge which rises in value very fast and consequently a large n is produced. It can be also shown mathematically that the larger the nf, the more nearly constant are the amplitudes mentioned secondary winding that the voltage induced in the latter tends to render the control grid still more positive, thereby causing a very rapid increase of current from the anode to the cathode of the tube. This effect is cumulative, increasing the positive potential of the grid still more until the current through the tube approaches the saturation point.

However, during the foregoing process a charge has been accumulating on the capacitor tending to make the control grid less positive and'so to decrease current flow through the primary winding. Thus thevoltage induced by the primary winding on the first-mentioned secondary winding falls to zero, thereby decreasin the positiveness of the grid potential which in turn results in a decrease in current flow in the primary winding and anode. This then reduces the voltage impressed by the secondary winding of the control electrode which decreases the current flowing through the primary winding and anode still more. Again the effect of the action is cumulative, and current flow from the anode source falls very quickly to zero.

The value of n being dependent of the harmonics, which is advantageous in many 1 applications of such devices. To produce the voltage pulses blocking oscillators of the conventional type using an electron tube having a primary winding connected in series with its anode, and having a resistor in series between 1 The foregoing action has produced a charge on the capacitor in series with the control electrode which impresses negative potential on the latter so that once current flow has ceased it cannot recommence, notwithstanding any positive voltage impressed by the alternating input signal, until the capacitor has had time to discharge itself which it does through a shunting resistor. The time constant of the capacitorresistor circuit is arranged so that the capacitor does not reach substantial discharge until after the end of the positive half-cycle of the alternating input voltage. The negative half-cycle of the input voltage of course has no tendency to render the electron tube conductive, and it is not until the beginning of the next half-cycle of the alternating input voltage that the electron tube is again rendered conductive to recommence the cycle of operation which has been described above.

By properly designing the relative magnitudes of the resistors, capacitors and transformer windings in ways well-known in the art, it is possible at kc. to make the duration of the voltage pulse from anode to cathode of the electron tube extremely small relative to the periodicity of the alternating input voltage.

However, at 1 me. it is impossible to use a conventional blocking oscillator as described above as the pulse widths are of greater length than a complete cycle of input voltage.

My invention is directed to a modification of the above described circuit. As the inductances of the transformer windings using an iron core are relatively high, I removed the iron core and designed the primary winding to have a natural frequency roughly 25 times higher than the input or triggering frequency. As the primary winding now-had a high Q (losses normally obtained in the iron core'load the circuit and thus destroy the Q), the blocking oscillator produced a damped signal having the natural frequencyof its primary winding. To avoid this-difiiculty, I critically damp out the resonant effects of the primary winding by using unilaterally conducting devices in the circuits magneticallycoupled to the primary winding.

One object of my invention is, accordingly, ;to produce an improved form of blocking oscillator generator.

Another object of my invention is to produce a generator of voltage pulses in which the .intervals between voltage ulses are shorter than were possible through the use of prior art circuits.

Still another object of myinvention is to pro-- duce a generator of voltage pulses which have an extremely short duration.

Still another object of my invention is to produce an improved type of generator of voltage pulses, of a type in which the duration of the individual voltage pulses is extremely shortcompared with the intervals which separate them;

Still another object of my invention is to produce a generator of voltage pulses which are of substantially shorter duration than those which were possible through the use of prior art circuits.

Other objects of my invention will become apparent upon reading the following description, taken in connection with the drawing, in which:

Figure 1 is a schematic diagram of the electrical circuit, which is a preferable embodiment of the principles of my invention;

sheath I8.

example, be a crystal rectifier, is shunted across the terminals of the secondary winding I2. A tertiary winding I5 is magnetically coupled to winding 2 and has its .terminals shunted by a unilaterally conducting device I6 which may be similar to the device I4. Winding I5 supplies current to the output circuit through a coaxial conductor having a core I1. and a grounded A terminatingresistor I9 interconnects the ground with the terminus of the core 11 which is remote from the tertiary winding I5.

- of tube I more positive so that current will begin Fig. 2 is a graph showing the wave form of the alternating input voltage impressed on my pulse generator; and

Fig. 3 is a graph showing the waveformat the output terminals of the electron tube circuit of Fig. 1.

Referring in detail to the drawingan electron tube I is shown as a pentode, but may be any type employing a control electrode, hasits, anode connected through a primary winding 2 to the positive terminal 3 of a direct current voltage source (not shown), the negative terminal of which is connected to the point 4, for example through ground. Thecathode of the tube I is connected to the point 4 througha resistori. The screen grid .of the tube I is connected to the point 4 through a by-pass capacitor 6 and through a resistor I to the positive terminal-3.

, capacitor 9 and resistorIIisconnected-to one end of a secondary winding I2 which is magnetically coupled to the primary winding 2. The other end of the secondary winding I2 is connected through a capacitor I3 to the control elec trode of thetube land to resistor 8. A unilaterally conductingdevice I4, which may for to flow fromits anode to its cathode through the primary winding 2. The secondary winding I2 is so poled that the voltage induced in it tends to render the, control electrode of tube I more positive; in other words this induced voltage makes the lower end of the winding I2 in the drawing positive relative to the upper end. The polarity of the rectifier I I is such thatit is non-conductive to current flow responsive to such a voltage; and the polarity of the rectifier I6. is likewise such thatthe voltage simultaneously: induced in the winding I5 can cause no currentfiow through crystal I6. Current thus .flows through *the blocking capacitor 5, secondary. winding I2, capacitor I3 and resistor 38 making. the control electrode of tube I positive, the inducedvoltage in winding I2 evidently aiding in this effect.

This=tends to further increase currentflow from the anode to the cathode of tube II, inducinga still further positive potential onrthelower end .of the winding IZrelative-to its .uppenend, and

therebyrendering the control electrode .ofatube I still more positive. -fIfhiscumulative efiectcontinues until current-flowirom anode tocathode of tube -I approaches; saturation, whereupon no further substantial increase of current through thewinding 2 is possible.

During all .of this time, acharge has been building up on the platesof the capacitor I3 rendering itslower platenegativeand acondition soon arises when thepotential ofithecontrol electrode oftube I begins to decrease instead of becoming more: positive. This tends to .decrease current flow from the anode of tube I through-winding 2-, and such current decrease through winding 2--tends to reverse the polarity of the voltageinduced-in winding i2 makingthe lower end of the latter negative relative to its upper end. -This causesthe control-electrodeof tube I to be lesspositive-which in turn decreases the current from-the anode to-the cathode. The decrease in current in winding :2 induces a voltage in winding-I2 which tends to make the control electi'ode-of tuloe I still less positive. This cumulative effect continues until the tube is no longer conducting current at which time the-condenser I3 is negatively charged. -If-it-were not for the presence of the crystal I4; the voltagethus induced in the winding I-2 might become'very substantiahbut thepolarity of the crystalsl i is such that it begins to conduct currentandprevents the inductance of the winding I! from resonating with its distributed capacity to produce local voltage oscillations. In short, the crystal l4 may be said to damp any tendency'g f the winding l2 to produce resonant oscillations; Current through the winding 2 can then fall negative under the efiect of the voltage charge in lthecapacitor l3 free from the disturbing efiect:.of such local resonance voltage oscillations. {iglsd as winding I2 is magnetically coupled to winding 2, the crystal l4 tends to damp local resonant voltage oscillations in winding 2. 1

Once current from the anode to cathode through tube I has fallen to zero, it remains at that value until the charge on capacitor l3. has time to leak away through resistors 8 andJ l; and these circuit elements are so proportioned that the complete leakage of this charge does riot occur until after the end of the first half-cycle of the positive voltage impressed by the input'signal wave shown in Fig. 2.

.It is then not until during the firs positive half-cycle of the input voltage wave that the charge on capacitor I 3 can disappeargjand the control electrode of tube I become positive enough to reinitiate current flow from the anodei tocathode of tube I, to be followed by a repetition of the cycle of events outlined above. -,-W;i ,nding 2 induces a voltage into winding l5, and toprefvent local: resonant voltage oscillation, crystal [6 is used to critically damp the output winding l5. As winding 2 is magnetically coupled towinding IS, the crystal 16 also tends to damp anyflocal resonant voltage oscillations in winding} 2. The output voltage across resistor I9 is of the wave form shown in Fig. 3 and it may be supplied to suitable clipper circuits well known in the art to reduce it to a substantially rectangular flat top pulse. 7

Using the above-described circuit, I have found it possible to produce rectangular pulses occurring at intervals of 1 microsecond which have a pulse width of only one-fiftieth of a microsecond. While this was sufilciently short for the purposes in which I was interested at the moment, my work convinces me that it would be possible by further reduction of certain circuit constants to reduce the pulse length to one-hundredth of a microsecond or less without unstabilizing the operation of the device. The practical needfor pulses of this short duration is evident from the consideration that in present distance determining devices employing square-topped voltage'pulses of the type under discussion, a pulse'width of onetenth of a microsecond corresponds to'j16 .4 yards of distance, and this error could be; reduced to 3.3 yards with something from one-' fifth ofthe above-mentioned voltage pulses having a width of one-fiftieth of a microsecond.

Another notable need for pulses of such short duration is in radio transmitters where it is designed to obtain the primary frequency by combining the outputs of harmonic generators. Thus for a radio transmitter operating on a frequency of several megacycles to thirty megacycles a device such as a one megacycle harmonic generator is imperative in its signal determining circuits.

I claim as my invention:

1. In combination with an electrical discharge tube having an anode, a cathode and a control electrode, a primary winding in' series with said anode, a secondary winding coupled to said primary winding and connected in series with a capacitor and said control electrode and a source of control voltage, a load circuit also coupled to said primary winding, and a unilaterally conductive device connected in shunt across the terminals of said secondary winding, the polarity of said unilaterally conductive device being such as to effectively short circuit said: secondary winding when the voltage induced therein by said primary winding tends to impressa negative voltage upon said control electrode.

2. In combination with an electrical discharge tube having an anode, a cathode and a control electrode, a primary winding in series with said anode, a secondary winding coupled to said primary winding in series with a capacitor and said control electrode and a source of control voltage, a tertiary winding coupled to said primary winding, and a unilaterally conductive device coupled across the terminals of each of said secondary and said tertiary windings.

3. In combination with an electrical discharge tube having an anode, a cathode and a control electrode, a primary winding in series with said anode, a secondary winding coupled to said primary winding in series with a capacitor and said control electrode and a source of control voltage, a tertiary winding coupled to said primary winding, and a unilaterally conductive device coupled across the terminals of each of said secondary and said tertiary windings, the polarity of said unilaterally conductive devices being such as to efiectively short circuit said secondary and tertiary windings when the voltages induced therein by said primary winding tend to impress a negative voltage upon said control electrode.

EVERETT C. DILL.

REFERENCES CITED The following references" are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,227,075 Geiger Dec. 31, 1940 2,250,706 Geiger July 29, 1941 2,265,620 Bahring Dec. 9, 1941 2,308,908 Bahring Jan. 19, 1943 FOREIGN PATENTS Number Country Date 750,044 France May 15, 1933 

